In recent years, homogeneous charge compression ignition (HCCI) engines from which excellent fuel economy and thermal efficiency can be gained have been drawing attention, and various researches have been conducted on such engines. In most HCCI engines, air-fuel mixture is created through mixture of fuel and air in an intake passage of the engine so that the air-fuel mixture can be supplied to a combustion chamber. Then, the temperature increases and the pressure rises in the air-fuel mixture contained inside the combustion chamber as the piston rises at the time of the compression stroke, so that the mixture spontaneously ignites. One obstacle that needs to be overcome in terms of putting HCCI engines into practice is that the engine operating range, which allows homogeneous charge compression ignition (HCCI) combustion to be stably controlled, is still narrow. In order to overcome the obstacle, there is a trend to implement HCCI combustion in stationary engines where the ordinarily used operating range is relatively narrow, for example, gas engines for GHPs (gas heat pumps). In addition, an engine where the combustion mode is switched so that HCCI combustion is carried out in the frequently-used middle rotation and middle load range, and spark ignition (SI) combustion is carried out in the low rotation range and the high rotation range, as well as in the low load range and the high load range, have also been proposed. Japanese Laid-Open Patent Publication No. 2000-220458 and Japanese Laid-Open Patent Publication No. 2004-293471, for example, disclose a method for controlling an HCCI engine where the combustion mode can be switched between the SI combustion and the HCCI combustion.
In Japanese Laid-Open Patent Publication No. 2000-220458, the throttle is gradually opened when the SI combustion is switched to the HCCI combustion, so that the amount of intake is higher at the time of the HCCI combustion than at the time of SI combustion. As a result, the air-fuel ratio at the time of HCCI combustion is higher than at the time of the SI combustion, so that the air-fuel mixture becomes lean at the time of operation through HCCI combustion, and the fuel economy and the thermal efficiency increase.
In addition, in Japanese Laid-Open Patent Publication No. 2004-293471, the throttle opening degree is temporarily fixed at a level between the opening degree at the time of the SI combustion and the opening degree at the time of HCCI combustion when SI combustion is switched to the HCCI combustion. As a result, pumping loss is reduced while the drivability is maintained, so that the fuel economy increases.
The throttle opening degree has a closing angle in the state of the SI combustion before the combustion mode is switched to HCCI combustion. Accordingly, there is a difference in pressure between portions on either side of the throttle within the intake passage. Specifically, negative pressure (intake negative pressure) reaches the portion of the intake passage on the side corresponding to the combustion chamber of the throttle, and substantially atmospheric pressure reaches the portion of the intake passage on the side opposite to the combustion chamber. When the throttle opens in the configuration described in the above Japanese Laid-Open Patent Publication No. 2000-220458 and Japanese Laid-Open Patent Publication No. 2004-293471, the flow rate of air in the vicinity of the throttle suddenly increases due to the difference in pressure. Accordingly, in the case where the fuel supplying section is located in the vicinity of the throttle, the air-fuel mixture becomes excessively lean. As a result, the engine misfires during the period in which the SI combustion is switched to HCCI combustion, and the engine may stall. In particular, in the case where a mixer or a carburetor is used to supply the air-fuel mixture, fuel is supplied at a point upstream from the throttle and in the vicinity of the throttle, and therefore, the air-fuel ratio (excess air ratio) in the air-fuel mixture is greatly affected by the opening and closing of the throttle.
The air-fuel mixture becoming lean is not a significant problem in the case where the excess air ratio in the air-fuel mixture required for HCCI combustion is high. However, in the case where HCCI combustion is carried out in an HCCI engine which provides a negative overlap period at the time of HCCI combustion and uses an internal EGR, it is necessary to supply a relatively rich air-fuel mixture into the combustion chamber. That is, it is necessary to adjust the air-fuel ratio so that the excess air ratio becomes low. In this case, lean air-fuel mixture as described above causes a significant problem. The negative overlap period is a period during which both the exhaust valve and the intake valve are closed when the piston is located in the vicinity of the top dead center in the exhaust stroke. In particular, in order to achieve high fuel economy and low amount of NOx emission, lean combustion is in some cases carried out at the time of the SI combustion. In this case, the excess air ratio sometimes becomes greater than that at the time of HCCI combustion, and thus, the above described problem becomes more conspicuous.
Thus, the amount of intake increases most suddenly when the throttle starts opening. Therefore, even when the amount of intake is adjusted using the technology disclosed in the above Japanese Laid-Open Patent Publication No. 2000-220458 and Japanese Laid-Open Patent Publication No. 2004-293471, it is difficult to suppress such a sudden increase in the amount of intake and prevent the air-fuel mixture from becoming excessively lean as a result.